In a process for manufacturing substrates such as silicon wafers, a process for cleaning substrates such as a dip type or a single-wafer type has been conventionally performed, in order to remove from the substrates organic substances, metal impurities, particles (fine particles), native oxide film, and the like that may cause defects in semiconductor devices.
In such processes for cleaning substrates, various types of cleaning methods are used to suit their purposes. In particular, when foreign substances such as particles are removed by a dip type cleaning method, a method is used in which a substrate is dipped in a cleaning liquid contained in a cleaning tank, and the cleaning liquid in which the substrate is dipped is irradiated with ultrasonic waves having a frequency around 1 MHz, referred to as “megasonics”. It is generally believed that the use of ultrasonic waves having a frequency around 1 MHz can increase the cleaning effect on submicron-size microparticles on the surface of a substrate, while reducing damage to the substrate.
The concentration of a gas dissolved in a cleaning liquid affects the removal efficiency for foreign substances such as particles. For example, when particles are removed from substrates by using ultrapure water as a cleaning liquid, and irradiating the ultrapure water with megasonics, the particle removal efficiency from the substrates is affected by the concentration of nitrogen dissolved in the cleaning liquid. More specifically, when the concentration of nitrogen dissolved in the cleaning liquid is within a prescribed range, the particle removal efficiency from the substrates is relatively high. Thus, theoretically, it is possible to effectively remove particles if the concentration of a dissolved gas such as the concentration of nitrogen dissolved in the cleaning liquid is monitored in the cleaning process, and the concentration of the gas dissolved in the cleaning liquid is controlled to be within a certain range.
Conventionally, a gas component contained in a fluid medium is introduced into a receptor through a polymer membrane, and the concentration of the gas component is calculated based on a change in thermal conductivity within the receptor (Japanese Patent Laying-Open No. 1991-176640). Thus, a method in which the concentration of nitrogen dissolved in a cleaning liquid is monitored using this measurement method is being conducted.